New transparent display system could provide wide-angle-view heads-up data

January 22, 2014

Photographs showing transparent screen (left) in comparison with a regular piece of glass (right). A laser projector projects blue MIT logo onto both the transparent screen and regular glass; the logo shows up clearly on the transparent screen, but not on regular glass. Three cups are placed behind to visually assess the transparency. (Credit: C. W. Hue et al./Nature Communications)

MIT researchers have come up with a new approach to transparent displays that could have significant advantages over existing systems for certain kinds of applications: wide viewing angle, simplicity of manufacture, and potentially low cost and scalability.

Transparent displays have a variety of potential applications — such as the ability to see navigation or dashboard information while looking through the windshield of a car or plane, or to project video onto a window or a pair of eyeglasses.

A number of technologies have been developed for such displays, but all have limitations.

The innovative system is described in a paper published in the journal Nature Communications, co-authored by MIT professors Marin Soljačić and John Joannopoulos, graduate student Chia Wei Hsu, and four others.

Many current “heads-up” display systems (such as Google Glass) use a mirror or beam-splitter to project an image directly into the user’s eyes, making it appear that the display is hovering in space somewhere in front of him. But such systems are extremely limited in their angle of view: The eyes must be in exactly the right position in order to see the image at all. With the new system, the image appears on the glass itself, and can be seen from a wide array of angles.

Other transparent displays use electronics directly integrated into the glass: organic light-emitting diodes for the display, and transparent electronics to control them. But such systems are complex and expensive, and their transparency is limited.

The secret to the new system: nanoparticles are embedded in the transparent material. These tiny particles can be tuned to scatter (to make the image look opaque) only certain wavelengths (colors) of light, while letting all the rest pass right through.

That means the glass remains transparent enough to clearly see colors and shapes in the outside world through the glass display, while a single-color image is clearly visible on the glass.

To demonstrate the system, the team projected a blue image in front of a scene containing cups of several colors, all of which can clearly be seen through the projected image.

The team’s demonstration used silver nanoparticles — each about 60 nanometers across — that produce a blue image, but they say it should be possible to create full-color display images using the same technique.

The three primary colors (red, green, and blue) are enough to produce what we perceive as full-color, and each of the three colors would still show only a very narrow spectral band, allowing all other hues to pass through freely.

“The glass will look almost perfectly transparent,” Soljačić says, “because most light is not of that precise wavelength” that the nanoparticles are designed to scatter. That scattering allows the projected image to be seen in much the same way that smoke in the air can reveal the presence of a laser beam passing through it.

Heads-up windshield displays

Such displays might be used, for example, to provide heads-up windshield displays for drivers or pilots that work full-size at any viewing angle.

Soljačić says that his group’s demonstration is just a proof-of-concept, and that much work remains to optimize the performance of the system. Silver nanoparticles, which are commercially available, were selected for the initial testing because it was “something we could do very simply and cheaply,” Soljačić says. The team’s promising results, even without any attempt to optimize the materials, “gives us encouragement that you could make this work better,” he says.

The particles could be incorporated in a thin, inexpensive plastic coating applied to the glass, much as tinting is applied to car windows. This would work with commercially available laser projectors or conventional projectors that produce the specified color.

The work, which also included members of the U.S. Army Edgewood Chemical Biological Center, was supported by the Army Research Office and the National Science Foundation.

Abstract of Nature Communications paper

The ability to display graphics and texts on a transparent screen can enable many useful applications. Here we create a transparent display by projecting monochromatic images onto a transparent medium embedded with nanoparticles that selectively scatter light at the projected wavelength. We describe the optimal design of such nanoparticles, and experimentally demonstrate this concept with a blue-color transparent display made of silver nanoparticles in a polymer matrix. This approach has attractive features including simplicity, wide viewing angle, scalability to large sizes and low cost.

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comments 1

So, am I right in thinking that anyone on the reverse side of the glass sees no display? Nothing at all?

“[...] while looking through the windshield of a car or plane, or to project video onto a window or a pair of eyeglasses.”

Would this really be suitable for eyeglasses? Can the imagery be made to appear to hover some distance infront of the viewer like Google Glass? (i.e. with no focussing issues)

In windshields, there could be the potential for some ne’er-do-well with a laser to cause all sorts of havoc.

Loads of potential, though!

There are also trials being conducted with silver nanoparticles set into hydrogel to create ‘holographic sensors’. These change colour in response to certain compounds to measure levels of glucose, alcohol, hormones, drugs, or bacteria.